Some electronic devices can be bent for a certain degree of curvature, but cannot be arbitrarily stretched or squeezed, such as a flexible screen. For such electronic devices, the design of a rotary shaft requires that an inner length of the rotary shaft to remain constant and an outer size of the shaft can be increased or shortened. As such, an electronic device arranged at the inner face of the rotary shaft can be bent.
Therefore, a rotary shaft with a constant inner length and a variable outer size is needed.